Greinar (Vísindafélag Íslendinga) - 01.01.1977, Side 73

71
ing in the uppermost 3-5 km thick crust (cf. Chapter 3).
This limitation of focal depth implies that the stress which
causes an accumulation of strain energy, and ultimately frac-
turing in the thin brittle “crust”, led to flowage of the under-
lying material (within the Oceanic Layer) at a temperature of
no more than 150°C (e.g. on transcurrent fault with normal
temperature gradient). Thus the shallowness of earthquakes
on ridge systems proves flow of material, which may be essen-
tially basaltic, and corresponding in depth and temperature to
similarly buried basalts of lower Tertiary or earlier age, which
are now exposed. All that seems necessary for flow, is a non-
hydrostatic stress, sufficiently adjusted to the plasticity de-
gree. This is just an extension into geologic time scale of the
experimental result that brittle material becomes ductile under
pressure, and breaks or flows in accordance with the inten-
sity of the non-hydrostatic stress. Actually, it has been known
for a long time, from the study of folded rocks, that any rock
may flow, when given sufficient time, i.e. when the applied
stress is below the breaking strength of the rock for a geo-
logic time interval.
Such flow of rocks is more than a rearrangement of mine-
rals, it means a deformation of the minerals themselves along
shear planes and many possible lattice planes.
The very existence of a Curie point, 500—800°C below the
crystallization temperature of the minerals in basalts, indicates
the weakness of the bonds within the magnetic domaines in
comparision with the lattice bonds of the minerals. Thus, we
could also expect plastic deformations within the domaines.
We now suggest that by a combination of the effect of magne-
tostriction and plasticity in seismic fracture, the magnetic
vector of the affected rock becomes aligned with the shear
movement leading up to the shallow earthquakes of the ridge
areas, and so creates the magnetic anomalies.
We may, in fact, have an example in the Stardalur anomaly
in Iceland (49).
This strong magnetic anomaly, reaching above 70,000 gam-